AC Milan owner Silvio Berlusconi has questioned the logic behind the club's decision to sign Mancini.
The Brazilian winger has joined the Rossoneri on a six-month loan deal from city rivals Inter Milan.
He was keen to move on after finding first-team opportunities hard to come by under Jose Mourinho.
Berlusconi, though, believes he could face a similar situation at AC Milan, with the club already well-stocked for creative talent.
"I don't understand it at all," he said.
"He's another winger while we maybe needed someone to finish off chances.
"He's not really played for two years, I'm not in agreement with the signing and I told [club vice-president, Adriano] Galliani."
Mancini is happy to have completed a switch, though, with the 29-year-old hoping to be offered the opportunity to get his career back on track after stagnating at Inter.
He said: "I have thrown away six months. I was close to leaving Inter last summer but Mourinho told me that he believed in me.
"Mourinho is good but I am now with (AC Milan coach) Leonardo, someone who understands football.
"I would gladly cancel the past year but now the time has come to turn the page, a new adventure begins for me."
Meanwhile, Leonardo has revealed that he also tried to land Mancini's countryman Keirrison during the January transfer window, but was beaten to the Barcelona starlet's signature by Serie A rivals Fiorentina.
"I am happy that Keirrison has arrived in Serie A," he said.
"Not so much because he joins Fiorentina and my team could face him, but because I am strongly convinced of his potential.
"I know that Milan tried to sign him but I have no regrets. I have a strong team.
"I'm just curious to see Keirrison play in Serie A."